1. Field of the Invention
The present invention relates to a control apparatus, system, and method for controlling a semiconductor-based corrosion and fouling prevention system.
2. Discussion of the Background Art
The annual cost of metallic corrosion in the United States economy is approximately $300 billion, according to a report released by Battelle and the Specialty Steel Industry of North America entitled “Economic Effects of Metallic Corrosion in the United States,” dated 1995, the entire contents of which is hereby incorporated by reference. The report estimates that about one-third of the cost of corrosion ($100 billion) is avoidable and could be saved by broader application of corrosion-resistant materials and application of best anti-corrosive practice from design through maintenance. The estimates result from a partial update by Battelle scientists of the findings of a study conducted by Battelle and the National Institute of Standards and Technology titled “Economic Effects of Metallic Corrosion in the United States,” the entire contents of which are hereby incorporated by reference. The original work in 1978 included an estimate that, in 1975, metallic corrosion cost the U.S. $82 billion (4.9 percent of the Gross National Product), and approximately $33 billion was avoidable because best practices were not used at the time.
Regarding aviation, corrosion and the magnitude of its associated cost and effect on safety is a leading concern of global aircraft manufacturers, airline companies, and passengers. In North America alone, aircraft industry corrosion costs exceed $13 billion a year. The impact is equally as great for government aircraft with, for example, the U.S. Air Force spending in excess of $800 million annually for aircraft corrosion control and repair. Corrosion, not design life, is the primary factor in the grounding and retirement of aircraft. The FAA has ranked preventing aircraft structural failure as a top priority for improving aircraft and passenger safety. Aircraft corrosion is linked to a significant number of mishaps, accidents, and plane crashes. The tragedy of the loss of human life aside, the FAA has calculated the monetary cost per passenger fatality at $2.7 million. Left undetected and/or untreated, corrosion undermines the integrity of an aircraft, increasing maintenance costs, and the risk to passenger safety.
Regarding marine vessels, interior and exterior hull corrosion and exterior hull surface fouling are major factors affecting ship operating costs and vessel life. Fuel expenses represent 35% to 50% of overall operating costs. Corrosion, fouling, and the associated exterior hull roughness and skin friction contribute up to an additional 50% to these costs due to the increased power requirement necessary to attain and maintain vessel cruising speeds. Corrosion damage to interior hull surfaces, its cumulative effects on structural integrity, and the cost of correction, not vessel age, are the major deciding factors in vessel retirement and can significantly shorten the useful life of a ship.
Regarding water towers, there are an estimated 150,000 to 200,000 municipal water towers in the United States. An average water tower has a surface area, inside and outside, and of 23,000 square feet and holds 310,000 gallons. These towers are particularly corrosion prone due to excessive condensation resulting from the storage of cool water. To maintain structurally sound water towers, municipalities refurbish tanks approximately every six years in coastal areas and every seven to nine years inland, with an average cost per water tower in excess of $100,000.
Regarding bridges, the National Bridge Inventory lists 575,413 highway bridges in the United States, with 199,277 of them described as structurally deficient or obsolete as of 1992. The Intermodal Surface Transportation Efficiency Act of 1991 authorized $16.1 billion over a period of 6 years for the Highway Bridge Replacement and Rehabilitation Program. The Transportation Equity Act for the 21st Century, signed in 1998, continues the program with the authorization of $20.3 billion over the next 6 years for bridge rehabilitation and replacement. The Federal Highway Administration and the Transportation Research Board estimate that 100 million square feet of bridge surface is coated annually. The square footage painted per year has been restricted due to the costs and time required for the removal and containment of lead based paints. As a result, many states have delayed bridge maintenance painting and only an estimated 1,500 steel bridges are painted annually. With current coatings lasting only 10 to 12 years, the backlog of bridge recoating continues to grow.
Regarding automotive concerns, corrosion issues affecting vehicle safety are a major problem for automobile manufacturers and consumers alike. According to the National Highway Transportation Safety Administration, between 1975 and 2001, over 25,000,000 vehicles have been officially recalled in the United States for corrosion related safety problems. In 1998 alone, Ford Motor Company recalled over 2,000,000 vehicles for safety related corrosion problems at a cost estimated to be in excess of $200 million.
A variety of methods for controlling corrosion have evolved over the past several centuries, with particular emphasis on methods to extend the life of metallic structures in corrosive environments. These methods typically include protective coatings, which are used principally to upgrade the corrosion resistance of ferrous metals, such as steel, and some nonferrous metals, such as aluminum, and to avoid the necessity for using more costly alloys. Thus, they both improve performance and reduce costs. However, such protective coatings typically have several pitfalls, including poor applicability to non-metallic structures that suffer from corrosion or fouling.
Protective coatings fall into two main categories. The largest of these categories is the topical coating such as a paint that acts as a physical barrier against the environment. The second category consists of sacrificial coatings, such as zinc or cadmium that are designed to preferentially corrode in order to save the base metal from attack.
Cathodic protection and coatings are both engineering disciplines with a primary purpose of mitigating and preventing corrosion. Each process is different: cathodic protection prevents corrosion by introducing an electrical current from external sources to counteract the normal electrical chemical corrosion reactions whereas coatings form a barrier to prevent the flow of corrosion current or electrons between the naturally occurring anodes and cathodes or within galvanic couples. Each of these processes provided limited success. Coatings by far represent the most wide-spread method of general corrosion prevention (see Leon et al U.S. Pat. No. 3,562,124 and Havashi et al U.S. Pat. No. 4,219,358). Cathodic protection, however, has been used to protect hundreds of thousands of miles of pipe and acres of steel surfaces subject to buried or immersion conditions.
Cathodic protection is used to reduce the corrosion of the metal surface by providing it with enough cathodic current to make its anodic dissolution rate become negligible (for examples, see Pryor, U.S. Pat. No. 3,574,801; Wasson U.S. Pat. No. 3,864,234; Maes U.S. Pat. No. 4,381,981; Wilson et al U.S. Pat. No. 4,836,768; Webster U.S. Pat. No. 4,863,578; and Stewart et al U.S. Pat. No. 4,957,612). Cathodic protection operates by extinguishing the potential difference between the local anodic and cathodic surfaces through the application of sufficient current to polarize the cathodes to the potential of the anodes. In other words, the effect of applying cathodic currents is to reduce the area that continues to act as an anode, rather than reduce the rate of corrosion of such remaining anodes. Complete protection is achieved when all of the anodes have been extinguished. From an electrochemical standpoint, this indicates that sufficient electrons have been supplied to the metal to be protected, so that any tendency for the metal to ionize or go into solution has been neutralized.
Recent work in the study of corrosion has found that electrochemical corrosion processes appear to be associated with random fluctuations in the electrical properties of electrochemical systems, such as cell current and electrode potential. These random fluctuations are known in the art as “noise.” About 20 years ago, scientists found that all conductive materials begin corroding as soon as they are produced due to electrochemical activity caused by impurities in the material. It was later found that this activity could be monitored using electronic instruments detecting the current generated, now commonly referred to as “corrosion noise.” Essentially, the greater the magnitude of this current, the “noisier” the material and the faster the rate of corrosion. For example, steel is “noisier” than bronze and corrodes at a faster rate. Researchers have begun to apply noise analysis techniques to study the processes of corrosion in electrochemical systems.
Riffe, U.S. Pat. No. 5,352,342 and Riffe U.S. Pat. No. 5,009,757, the contents of each of which being incorporated herein by reference, disclose a zinc/zinc oxide based silicate coating that is used in combination with electronics in a corrosion prevention system. The zinc/zinc oxide particles in the coating are disclosed as having semiconductor properties, primarily a p-n junction at the Zn—ZnO phase boundary. When reverse biased, this p-n junction is described as behaving as a diode and inhibiting electron transfer across the boundary. This restriction limits electron transfer from sites of Zn oxidation to the sites of oxygen reduction on the ZnO surface. Effectively, there is increased resistance between the anode and cathode of local corrosion cells and corrosion is reduced.
On average, the Zn—ZnO based junction will be reversely biased due to the potentials associated with the oxidation of Zn at the Zn surface and the reduction of O2 at the ZnO surface. However, significant stochastic voltage fluctuations occur. These voltage fluctuations cause the junction to episodically become forward biased. When forward biased, electron transfer across the junction increases and there is an acceleration or “burst” of the oxidation of Zn and reduction of O2. Effectively, there is a short circuit between the anode and cathode of local corrosion cells and corrosion is enhanced.
The Riffe patents disclose attachment of a fixed value capacitor in the electrochemical circuit of the corrosion prevention system. However, as recognized by the present inventors, there is no recognition of the desirability of controlling the level of capacitance nor any method suggested for determining how to dynamically change the value of capacitance needed to effectively prevent corrosion in any given structure or an optimal way to determine the value of the capacitance needed.
Regarding anti-fouling, marine objects are degraded by barnacles, zebra mussels, etc. that, once attached, must be mechanically removed. Low-cost, non-mechanical, and environmentally friendly removal/prevention of marine fouling is desirable. This has led to research in anti-fouling toxicity. Toxicological studies have established the fact that “poison” must be operationally defined and in so doing, a compound's “toxicity” is frequently defined in terms of an amount or concentration of a compound that produces either death or disease. Accordingly, in an assessment of the relative toxicity of an element or compound, the concentration should be considered. Many metals are known to be “toxic.” However, a metal labeled as toxic is, at the same time, “essential.” Copper, a well-known anti-fouling agent, falls into this category. Without copper, life as we know it cannot exist. Copper is an essential part of certain enzymes that play critical roles in growth, reproduction, and metabolism. Unfortunately, at least for those wanting to use copper as an anti-foulant, low concentrations, measured in parts per million, are toxic, especially to aquatic organisms whose bodies are entirely bathed in their liquid environment. Tin is not an essential metal in biology, but organic tin compounds are particularly good anti-fouling agents. Unfortunately, levels of these or organotin compounds beginning at parts per billion levels are toxic to non-target species. In addition, the organotins accumulate in fatty tissues and are “magnified” by the food chain, having increasingly adverse affects on top of the chain animals, like humans.
Zinc is an essential metal in biology but it does not, like copper, fall into the category of a heavy metal. Toxic levels of zinc are significantly higher than copper and zinc finds its way into many environmentally acceptable products and materials. Duke University scientists discovered that the anti-fouling properties associated with the coatings of Riffe, U.S. Pat. No. 5,352,342 and Riffe. U.S. Pat. No. 5,009,757 came from zinc toxicity of the coating. These scientists simultaneously determined that the levels of zinc release were of such a low level that they would not produce toxicity in the marine environment. They also noted that zinc is not a metal that is magnified in the food chain. Accordingly, it is possible to use zinc ions as a toxic, anti-marine fouling agent.
One drawback to previous corrosion preventive methods, such as that of Riffe disclosed above, is the relative inflexibility of color selection available for the silicate based coatings disclosed therein, with the only color readily available being gray. While this is acceptable in most marine and structural uses, there is a need for corrosion preventive coatings that are non-sacrificial and which can be provided in a range of colors for use as paint substitutes, particularly in the automotive and transportation industries. These and other drawbacks are largely overcome with the semiconductor coatings and related systems of Dowling's U.S. Pat. Nos. 6,325,915, 6,402,933, and U.S. application Ser. No. 09/887,024 filed on 25 Jun. 2001, the entire contents of each hereby incorporated by reference. The semiconductive coating and system of the Dowling patents and application can be used with a variety of conductive substrates to provide an array of interesting properties. With the semiconductor always being a material less noble than the substrate on which it is applied, the coating stabilizes the potential of the protected material. The electrons produced by the electrochemical activity are transferred from the protected substrate to the semiconductor of the coating or, simply, the corrosion noise is transferred from the protected material to the coating.
FIG. 1 is a representation of electrochemical noise present in untreated metal 101 the randomly fluctuating voltage is measured and displayed as waveform 102 (shown as a sawtooth waveform, but an actual waveform would have broader band components and would be stochastic in nature).
FIG. 2 shows the effect of applying a semiconductive protective coating on a metal surface so as to prevent corrosion and fouling where the coating 210 comprises a material less noble than the metal 201 it is protecting. Because the coating 210 is less noble than the metal 201, it subsumes the electrochemical noise 211 that would be present in the metal but for the coating this result is displayed 202 as a flat waveform in the metal. Individual semiconductor particles within the coating 250 are responsible for the anti-corrosion properties of the coating.
FIG. 3 is a representation of a layered semiconductor/metal composition. When doped with zinc, the anti-corrosion capabilities of the semiconductor material for steel (ferrous alloys) results from the establishment of a potential due to Zn oxidation and oxygen reduction, referred to as “corrosion potential.” In this respect, the system acts as a conventional sacrificial anodic material with iron oxidation suppressed at the potential established by the Zn oxidation. However, Zn oxidation in a semiconductor is significantly reduced or passivated, with a reduction of the corrosion potential, resulting in the extreme long life of the coating. The passivation is a result of a combination of the varistor-like behavior of the Zn/ZnO boundary and an associated filter's ability to maintain a potential difference across the boundary, such that the boundary has a high electrical resistance. A semiconductor particle 250 is comprised of two regions: a P-type region 320 and an N-type region 310 with a junction 330 that behaves as a varistor with electron flow 302 between the two regions. When using zinc, the zinc particles are covered by a zinc oxide layer with the various oxide coated particles surrounded by a conductive binder. The boundary of the P and N semiconductors in the semiconductive coating acts as a varistor (back to back diodes) that controls the flow of electrons between them. Proper application of a current to the semiconductive coating, connected to the protected substrate, stabilizes the potential at this boundary. This slows the rate of electron transfer from the P to the N semiconductor, reducing its rate of corrosion by a factor of 103, yielding an extension in the life of the semiconductive coating that can exceed the design life of the treated object.
Varistors (variable resistors) have highly non-linear electrical characteristics and are functionally equivalent to back-to-back diodes. In a voltage limited region, the “switch region,” they pass only a leakage current. When the voltage magnitude exceeds the switch voltage, for instance during a transient, the varistor becomes highly conducting. Varistors are commonly based on ZnO. FIG. 4 is a graph representing the current voltage relationship for varistor, within which an axis representing voltage 1101, an axis representing current 1102, and a curve representing current 1103 over a range of biasing voltage are displayed. The range between −Vb 1110 and Vb 1107 represents the voltage region 1104 in which the varistor behaves as a switch. The point along the curve labeled IL 1105 is the point along the curve that corresponds to leakage current—that is, the small level of current that flows through the varistor even when the varistor is biased to behave as an open switch. The point labeled VN 1106 is the point along the curve that represents the switch voltage; in other words, the highest positive voltage value that corresponds to the switch region 1104 of the varistor. The point labeled VB 1107 represents the breakdown voltage of the varistor, where biasing voltages greater than VB cause the varistor to behave as a node. The point labeled negative IL 1108 represents the point along the curve that represents the negative leakage current. The point labeled −VN 1109 represents the point along the curve that represents negative switch voltage; in other words, the most negative voltage of the range representing the switch region 1104 of the varistor. The point labeled −VB 1110 represents the negative breakdown voltage.
The above-identified Dowling patents and application are at least directed to systems and devices for controlling corrosion comprising semiconductive coatings and a corrosive noise controlling system that includes a filter. In the case of the pending Dowling application, the corrosive noise controlling system includes an adjustable filter which may be adjusted based on feedback signals corresponding to the corrosive noise present in the coating.
The performance of the corrosive noise reducing system of the Dowling patents and application varies in accordance with the system's internal filter, which in its simplest form is essentially a capacitor. The Dowling patents and application also disclose combining the semiconductive coating with various passive and active filters. In the Dowling patents and application, the semiconductor coating acts somewhat as a resistor, which is in parallel with the system's internal filter. A summary of filter basics, such as how to implement a high-pass or low-pass filter, is found in Microelectronics Circuits, Fourth Edition, Sedra & Smith, Oxford University Press (1997), the entire contents of which are hereby incorporated by reference.
FIG. 5 is a graph of corrosion potential versus time with various filters. The horizontal axis 401 measures time in days while the vertical axis 402 represents potential relative to the semiconductor element measured in milli-volts. During an experiment directed to determining optimum filter characteristics for various corrosion environments, measurements were taken for seven systems at three points in time. The measured potential for each of seven filter configurations were recorded for those three samples and are indicated by various symbols listed in the legend. The graph displays the various results for each of the seven filters at the sampling points indicated from 410 through 430.
Electrochemical corrosion can be viewed schematically in terms of an equivalent circuit. Typically, the semiconductive material is doped with zinc. Thus, the simple equivalent circuit shown in FIG. 6 relates to the case of Zn oxidation. The anodic reaction occurs on the Zn and the cathodic on the ZnO. Note the Zn/ZnO boundary represents a varistor in the circuit. If the potential difference generated by the Zn/O2 redox couple falls stably in the switch region, the Zn oxidation is inhibited (or passivated) by the high resistance of the boundary. However, over the past decade, it has been demonstrated that there are self-generated electrochemical potential fluctuations, “electrochemical noise” associated with corrosion. As a result, even though the Zn/O2 potential may be in the switch region, there are likely to be fluctuations that drive the potential difference into the highly conductive region and allow electron flow and hence Zn oxidation.
The present inventors recognized that this is a way to passivate Zn so as to remove or filter the electrochemical noise. Removal of this electrochemical noise is via the filter, which in its simplest form, is a capacitor. The filtering effect maintains the potential across the Zn/ZnO boundary in the switch region and Zn oxidation is reduced and the life of the coating is increased. However, it is to be appreciated that the low pass filter may be augmented with passband (or notch) filters to selectively attenuate other frequency bands depending on the material being protected.
FIG. 6 shows an equivalent circuit diagram for the system of the Dowling patents and application. This figure abstracts the behavior of the system into a representative electrical circuit based on the electrochemical nature of metal corrosion processes. Specifically, corrosion can be modeled as a fluctuating voltage source, the metal's inherent resistance can be represented, the anti-corrosion coating can be modeled as a varistor, and the noise filter can be modeled as a capacitor. By placing these modeled elements in a circuit diagram, the noise and filter components of Dowling can be more clearly conceptualized using electrical circuit analysis.
Within the representational circuit is a solution resistance 801 which represents the inherent resistance of the system in series with the galvanic electrode potential at the anode 802 which corresponds to the ionization process of zinc and the galvanic electrode potential at the cathode 803 which corresponds to the chemical process producing water. Also present and connected in series with the circuit are two noise sources 804, one of which is interposed between the galvanic electrode potential of the anode and the Faradaic impedance of the anode 805 and another interposed between the galvanic electrode potential at the cathode 803 and the Faradaic impedance of the cathode 806 placed in series between the Faradaic impedances of the anode and cathode are the zinc oxide varistor 807 and the noise filter 808. The varistor and noise filter act to reduce the occurrence of voltage fluctuations which can induce corrosion. The noise filter 808 may be active, passive, or both and, by selecting a node in the circuit to be designated common potential 810, the filter 808 can attenuate high frequencies in the circuit due to the corrosion noise.
The substrate on which the semiconductive layer is placed may be conductive or non-conductive. Conductive substrates can be metallic or non-metallic. Non-conductive substrates can be any material that acts as an insulator, such as a silicon wafer or other non-metal substrate. The production of such non-conductive or conductive substrates in the art of semiconductor chip manufacture is well known to one of ordinary skill in the art.
The corrosion noise reducing system of the Dowling patents and application provides a means for preventing corrosion of a conductive structure susceptible to corrosion by coating the conductive structure with a semiconductive coating and connecting the resulting coated structure to a passive or active electronic filter so as to minimize the corrosive noise in the coating. The electronic filter has a filter response such that it attenuates the high frequency spectral content of the corrosion noise. This is achieved by connecting a filter, having an impedance characteristic in the form of a low pass filter (possibly augmented by notch filters) across the material being protected. Furthermore, depending on the material and the application, possibly other frequency bands may selectively be attenuated so as to reduce corrosive and/or antifouling effects. The filter can be a passive filter or an active filter. In either case, the filter attenuates the higher frequency voltage fluctuations. The junctions present in the semiconductor coating then maintain a reverse bias. The time-averaged electron flow from the anodic to the cathodic domains in the semiconductive coating is then reduced and the coating is effectively passivated.
With the filter engaged to the circuit equivalent of the corrosion process, the noise signal can be dissipated as shown in FIG. 7, where a metal surface 501 is covered by a protective coating 510 connected to a filter 508 so the metal has a significantly attenuated noise electrostatic 502. The filter 508 acts either as a standalone low pass filter or possibly in combination with filters having impedances in the form of bandpass and/or notch filters to reduce the high frequency corrosive noise 522. Effectively, the filter dissipates the energy associated with the higher frequencies in the electrochemical noise signal. Attenuation of the high frequency spectral contents of the electrochemical noise will significantly reduce the corrosion process by inhibiting the voltage fluctuations across the varistor outside the switch voltage (Vn).